The expression “extreme ultraviolet” is understood to denote electromagnetic radiations having a wavelength of less than about 150 nm, and more specifically the wavelength of 13.5 nm which will be of more particular interest subsequently.
In what follows, reference will be made to the optical axis defined by the optical source and the image point which is formed thereof by the collector device, and the terms “front” and “rear” will be employed with reference to the direction from the source to the image point on this axis.
Collecting the luminous flux delivered by a quasi-pointlike source of electromagnetic radiation such as an arc lamp is not an easy thing to do when one wishes to collect the largest possible quantity of photons emitted by the source in the surrounding space.
This concern becomes all the more acute when the equipment into which the source and the luminous flux collector device are integrated must be optimized in relation to the useful luminous flux parameter for the method implemented.
Optical microlithography is typically a field for which expensive equipment must produce the largest quantity of silicon microchips per hour by irradiating a photosensitive resin deposited on silicon wafers from which the microchips will thereafter be sliced.
A few years ago, optical microlithography used sources of the mercury arc lamp type, selecting one of the spectral lines of said mercury arc, typically at wavelengths of 435 or 365 nm.
Currently, to etch ever finer patterns, microlithography machines use shorter wavelengths, typically 248 nm and 193 nm, delivered by power lasers of the KrF or ArF type. Collection of the luminous flux is facilitated thereby.
But the requirement for etching yet finer patterns on microchips leads to the envisaging of even shorter wavelengths provided by new types of sources.
EUV radiation at the wavelength of 13.5 nm is currently envisaged for the next generation of optical microlithography apparatus, forming the subject of significant developments in all the technologies involved in the various steps of the optical microlithography process:                making a mask receiving an object pattern intended to be projected by reduction onto the silicon wafer,        making high-resolution optical projection devices designed to carry out said projection,        making optical illumination devices designed to shape the flux collected so as to inject it onto the mask and the projection optic,        making optical collector devices that pick up the maximum luminous flux provided by the source,        making powerful, stable and reliable EUV sources,        making machines (scanners) for manipulating and positioning the silicon wafers and the masks facing the optical devices and ensuring the best positioning precision and the best execution speed.        
Significant research has been carried out on electromagnetic radiation sources situated in the extreme ultraviolet region. These sources belong to two principal categories, namely sources of the laser pulsed plasma type and sources of the capillary discharge plasma type.
The main characteristic of these two types of sources is to have a quasi-pointlike light emitter element, that is to say of a size which is typically of the order of a millimeter, but capable of radiating into a significant angular space of possibly as much as a hemisphere, or indeed more.
Another characteristic of EUV sources in view of the powers necessary for microlithographic applications is that they deliver many useful photons, but also other undesirable elements:                photons situated outside the useful spectral band,        ions and other particles ejected at high speed which may erode the surface of the optical elements placed too near the source,        diverse debris that is also deposited on the surface of the optical elements of the luminous flux collector device,        an intense thermal radiation, partially absorbed by the optical elements of the luminous flux collector device which then undergo undesirable heating.        
Thus, the resistance of the luminous flux collector device to these numerous attacks is a major critical element which must be solved in order to permit the effective deployment of EUV technology in the field of microlithography. One of the objects of the present invention is to provide an innovative response to this problem area.
EUV electromagnetic radiation, which is of short wavelength, is easily absorbed by the surfaces on which it is made to reflect.
To improve this reflectivity, it is known                to reflect the radiation with the aid of a limited grazing reflection of 10 to 15° maximum (with respect to the reflecting surface or to its tangent plane) to obtain a reflectivity of 80% and more on metals such as ruthenium or molybdenum deposited on the polished surface of the reflector mirror,        to reflect the radiation at quasi-normal incidence on the polished surface of a reflector mirror on which a set of thin layers has been deposited and which, through the physical phenomenon of constructive interference, afford a reflectivity of the order of 65 to 70%; however, one drawback is that, when the incidence of the radiation strays too far from normal incidence, the reflectivity decreases even if the thicknesses of the various layers are optimized to take account of the exact incidence of the rays.        
The quantity of luminous flux which is collected and actually transmitted to the following optical modules of the microlithography machine is another critical element. Any percentage gain in the luminous flux collected with respect to the luminous flux emitted by the source allows a production rate gain and/or reduction in the requirement in terms of effective power of the source, therefore a reduction in its power of attack on the optical elements of the collector and thus a gain in terms of useful lifetime of the equipment as well as in the effective cost of the installation.
A simple and effective flux collector that is commonly used, in particular in cinema projectors, consists of an axisymmetric ellipsoidal collector mirror in which the source is placed at one of the foci, the image point being formed at the other focus. This device has the advantage of being simple and limited to a single component. However, it cannot be transposed to the EUV domain for several reasons. A first reason is that the flux collected is formed with the radiation emitted by the source toward the rear, then reflected toward the front, that is to say it “crosses” the source and is focused in front of the latter; now, EUV sources are complex and bulky devices (typically 30 to 40 cm in diameter for a luminous source point of about 1 mm): a source such as this placed at the focus of the ellipsoidal collector would block the flux reflected by the mirror. A second reason is to do with the fact that the angles of incidence of the rays on the useful surface of the collector vary greatly according to the location of the reflection on the surface of the mirror, from quasi-normal incidence at the center of the ellipsoidal mirror to almost grazing angles of incidence near the edge of the mirror; treatment of the reflecting surface so as to ensure reflection in the EUV region must therefore entirely change in nature between the center and the edge of the mirror, which is very complex, or indeed impossible, to achieve at least under acceptable economic conditions. Therefore, despite its advantage in principle, this solution cannot be employed.
The mode of EUV flux collection commonly adopted currently, inspired by the design of X-ray telescopes, consists in utilizing mirrors with grazing incidence and in using reflectors of quasi-cylindrical shape arranged one after the other and combined by inserting them into one another, so that the luminous flux collected is deflected by successive reflections at grazing angles of incidence and consists of successive angular rings with blocked-out intermediate zones in which the flux is lost. Another drawback of this known technique is that, to pick up a significant portion of the light emitted by the source, the first elements must be brought near to the source: in devices made on the basis of this principle, the first optical elements are placed at a distance of 10 to 15 cm from the source and attacks by erosion, contamination or heating then become very serious; the effective lifetime of the collector device remains limited (see for example document US 2005/094764).
Document EP 1 469 349 discloses a collector device for EUV flux based on the use of a concave mirror and of a convex mirror according to a “twin mirror telescope of the Cassegrain type” arrangement. This arrangement suffers from insufficient luminous collection effectiveness and does not constitute an appropriate response to the problem posed, in particular on account of the high angles of incidence of the rays arriving at the second convex mirror.
Document WO 2005/031748 presents a collector arrangement based on a large concave mirror which forms an image of the source shifted laterally with respect to the source; thereafter the luminous flux is reflected forward. This known device suffers from a lack of luminous collection effectiveness due to a limitation of the optical extent collected by the large mirror and due also to too close a proximity of the secondary mirror to the source itself, which hampers the course of the rays, as well as to the presence of various devices intended for capturing or deflecting the particles or debris emitted by the source.
Document US 2004/223531 presents an EUV flux collector device consisting of an elliptical mirror giving an image of the source point and supplemented with a second, annular mirror picking up the additional flux and returning it to the same image point by virtue of a second reflection on an “axicon” type element placed behind the source. The major drawback of this known solution is that it is inappropriate for sources of the “Capillary Discharge Plasma” type, since the optical path passes through the source, precisely where the optical, mechanical and electronic components operating the source are housed. This known device therefore has limited conditions of use which may be inappropriate within the context of a microlithography installation.
Document JP 2005/109502 presents a device comprising a collector which conjugates the source point and the image point of the source and which is mounted on a moving support. This known device does not provide an effective response and does not obviate the resulting drawback of the high degree of deformation of the optics under the thermal flux of the source.
In a general manner, none of the currently known devices is capable of collecting an appreciable fraction of the luminous flux emitted by the source, and their effectiveness remains limited to a few percent and/or they are arranged in a manner such that some at least of their components are rapidly impaired and/or they are too cumbersome for the application more specifically envisaged in the field of microlithography.
The constraints inherent in integrating the collector device into a microlithography machine operating with an EUV source should also be remembered: impossibility of implementing optical lenses, no material being transparent for these wavelengths, and necessity of implementing reflecting optical components only; bulky source which requires that the entire optical collection hardware be situated in front of this source; very congested environment which leaves only a restricted free space for setting up the optical collection components; focusing distance (typically of the order of 1.20 m) which is not capable of being appreciably lengthened because of the constraints of general congestion within the machine.
Finally, it is apparent that, in the device indicated in the preamble, the central part of the light cone emitted by the source is not picked up by the main stage of the collector device which has just been described. Although not critical since this central part represents only a small proportion of the total flux emitted by the source, it is however beneficial to seek to collect the maximum of flux emitted by the source, so as to improve the performance of the device to the maximum.